1. Field of the Invention
The present invention relates to a digitally controlled grinding machine with a rest apparatus for supporting a workpiece, rotatably carried on a workpiece support device, against grinding resistance.
2. Description of the Prior Art
Heretofore, there have been used rest apparatus which are unable to perform an automatic compensation for wear of a pair of rest shoes, and it has been required that an operator frequently readjust the positions of the rest shoes.
In general, the press of a pair of shoes of a rest apparatus against a circumferential surface of a workpiece must be carried out after the circumferential surface is roughly ground to have its roundness and surface roughness improved up to a certain grade, and each of the shoes must be advanced to the position where they are able to completely remove from the workpiece a flexing caused by grinding resistance.
To this end, in a certain kind of the prior rest apparatus, as disclosed in U.S. Pat. No. 3,967,414 to T. Tamesui et al, a pair of rest shoes are advanced by a predetermined amount in a first step after a rough grinding feed of a wheel head is discontinued based upon a first size signal from a workpiece measuring device or a signal issued when the rough grinding feed attains a feed amount being preset in a setting device. Following this, the advance feed of the wheel head is restarted, and after the advance feed of the wheel head is discontinued based upon a second size signal from the measuring device, a second step is initiated to advance the shoes. This second step is then terminated when a third size signal is generated from the measuring device.
Since the feed of the shoes in the second step is controlled based upon the third size signal obtained as a result of measuring the workpiece in process, it had been understood that wear of the shoes, even if any, does not have any effect on grinding accuracy of the workpiece. However, there was observed a phenomenon that the geometric accuracy such as roundness of workpieces finished under the use of the prior rest apparatus is deteriorated in proportion to increase of the workpieces in number, and therefore, it is deduced that the deterioration of the geometric accuracy is measurably caused by wear of the shoes.
Directing attention now to the feed of the shoes in the first step, if some wear is effected on the shoes, the pressing surfaces of the shoes are retracted by the wear amount, and the shoes, when at their advanced end positions are advancing by a predetermined amount, are unable to press the workpiece back onto an ideal rotational axis thereof, whereby a flexing caused by grinding resistance remains in the workpiece as indicated at P2 in FIG. 3. It is therefore deduced that the retention of the flexing results in deterioration of the roundness of the workpiece at a stage that passes from a rough grinding step to a fine grinding step, that, even when the workpiece is ground to a finished size, the deteriorated roundness is left uncured, thus causing the workpiece to take the form of an ellipse, and that the tendency to take the form of an ellipse is developed in proportion to the growth of wear of the shoes.